In This Century, Changing Your Oil Still Matters

The Steering Column.

December 2004
By
CSABA CSERE

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Over the decades, new technologies have reduced the needs of automotive maintenance. Grease jobs for chassis lubrication are a thing of the past. So are tuneups. Spark-plug changes are as long as 100,000 miles apart. And oil-change intervals have also been extended. When I started driving in 1967, oil was changed every 3000 miles. Now, 7500 miles is almost standard. Our last Porsche 911 long-term car only required an oil change every 15,000 miles.

It's enough to make some drivers forget entirely about maintenance. But despite their onboard microprocessors, polished wood, tanned leather, and gleaming paint, cars are still machines full of parts that rotate and rub and survive only through lubrication.

Recently, Volkswagen sent a letter to owners of 1998-through-2004 Passats equipped with the 1.8-liter four-cylinder turbocharged engine informing them that it was extending the warranty on the engine to cover problems caused by oil sludge. Owners of Audi A4 1.8Ts received the same letter. Chrysler 2.7-liter V-6 engines have had similar problems, as have some Toyota V-6 and four-cylinder engines. The number of engines affected is small in each case, but in this computer-controlled age, oil sludge sounds like a Neanderthal problem.

Sludge is a thickening of the engine oil until it reaches a jelly-like consistency. When the crankcase of your engine starts to accumulate sludge, the thickened oil finds its way into the passages that carry pressurized oil to the engine's bearings and other lubricant-needy parts. Since many of these passages are about the size of a small soda straw and sludgy oil has the consistency of an icy milkshake, the oil flow can become restricted. If that happens, components such as valve lifters soon start wearing rapidly and operating erratically.

There are several ways for sludge to form in your engine. Water gets into the crankcase through condensation and blowby from the combustion chambers. Water mixed with oil takes on the consistency of thick, pus-colored foam. You'll know it if you see it.

Fortunately, small quantities of water evaporate from the crankcase as the engine warms up and the oil temperature gets above 212 degrees, which it does after a few minutes on a freeway. However, if you only drive short trips, your crankcase never gets hot enough to evaporate any accumulated water, and in such an engine, sludge formation seems certain in its future.

Sludge can also form if the oil gets too hot. At temperatures above 250 degrees, nonsynthetic oil starts to oxidize. Such oxidation thickens the oil and produces acidic byproducts. The process accelerates when the oil temperature hits 300 degrees, and the result is something more akin to tar than oil.

Even in 24-hour races, I've never seen an indicated oil temperature approach 300 degrees. But it's still possible for engine oil to experience near-300-degree temperatures somewhere in the engine. If that takes place, the oil will oxidize, a little bit at a time, which is why good engine design seeks to eliminate such hot spots.

The final cause of sludge is simply the passage of time. Tiny particles of soot get by the piston rings and end up suspended in the oil. So do various acids formed by the products of combustion. Oil contains additives designed to neutralize these contaminants, but eventually, enough of this foul stuff builds up to compromise oil's lubricating qualities. Manufacturers conduct tests to determine this contamination and specify oil-change intervals accordingly. And therein seems to lie the cause of the sludge problems.

According to Toyota, many of the company's sludge-damaged engines were in cars with 30,000 to 40,000 miles that had recently come off lease. Some of the engines still carried the original oil filter that was screwed on when the car left the Toyota assembly line, leading Toyota to conclude that some lessees were remiss about changing their oil.

Other diligent owners changed their oil, but as it turned out, not often enough. Like most owner manuals, Toyota's specified oil changes at 7500 miles. But its manuals also specified 3750-mile changes for cars in "severe" service. "Severe" service applies to vehicles used on short trips, especially in winter, situations where the engine never warms up. It also involves towing and stop-and-go driving in heavy traffic, especially in hot climates, which elevates oil temperature. These are exactly the conditions that promote sludge formation.

So how is an owner to determine the threshold between "normal" and "severe" service? Toyota concluded that an owner couldn't be expected to know and has since revised its oil-change schedules for all its vehicles to 5000 miles.

That's the same interval VW specifies for the 1.8T engines in the latest sludge case. Although these engines are also used in Audi TTs and VW Jettas and Golfs, the sludge problem seems confined to the VW Passat and Audi A4, which are heavier than the other cars and whose engines are mounted longitudinally rather than transversely.

In these 1.8T applications, the oil seems to run hotter and won't last much beyond the 5000-mile change interval. As a result, for these engines Volkswagen now recommends one of several synthetic oils that meet a specific VW 502.00 standard. Furthermore, VW's extended sludge warranty only applies to owners who can demonstrate that they changed their oil according to the factory schedule.

The best solution to the sludge problem seems to be the oil sensors that have been used for years on all BMWs, along with more than 90 percent of GM vehicles. These devices are not really "sensors" that somehow analyze oil quality; they are software programs that record cold starts, oil temperature, and a count of cylinder firings. The systems then infer the oil's condition based on its measured "experience." They can call for oil changes in fewer than 5000 miles and more than 10,000 miles, depending on driving conditions.

Until oil "sensors" become universal, you are well advised to change your oil according to factory recommendations and adopt the "severe" interval if you have any doubts about your driving conditions.

And check your oil regularly. It's not only important to keep the crankcase full, but your dipstick can reveal the moisture-induced dirty foam or the heat-formed tar that indicates a sludge problem, and a potentially expensive repair, in your engine's future.

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